Former New York City Mayor Michael Bloomberg is reportedly planning to run for president in 2020 as a Democrat, setting up the potential for a billionaire-on-billionaire showdown.

Mr. Bloomberg, who mulled presidential runs in 2008, 2012 and 2016, has told confidants that he plans to challenge President Trump’s re-election bid, according to a Thursday report in the Times of London.

“Mike Bloomberg told me he is going to run in 2020,” a source told the Times. “He has the money to see it through while other candidates knock themselves out.”

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The latest Bloomberg speculation comes after the New York Post reported in June that Mr. Bloomberg told guests at a fundraiser, “Before you leave I want to get your cellphone numbers because I’m thinking of getting the band back together.”

The media and financial mogul, whose worth was estimated by Forbes at $52 billion, has maintained a high profile as a Trump critic, particularly on the issues of gun control and climate change.

Mr. Bloomberg, 76, has switched his party political affiliation at least three times in the past 20 years, moving from Democrat to Republican for his 2001 mayoral campaign and then registering as an independent, but has lately shifted back to the Democratic side.

He pledged to spend $80 million to elect candidates in the midterm elections, a total expected to surpass even that of Democratic megadonor Tom Steyer, the party’s top donor in the past two cycles.

Other Democrats seen as potential contenders for the party’s presidential nomination include Sens. Kamala Harris of California and Elizabeth Warren of Massachusetts. Sen. Bernard Sanders, Vermont independent, who finished second for the Democratic nod in 2016, is also seen as a likely challenger.